Polyurethanes represent a broad family of polymers with great commercial and industrial importance. While these materials can be formulated to have a wide range of material properties, polyurethanes are well known for their abrasion resistance, toughness, flexibility, impact resistance, tear strength, and chemical resistance. Principal applications include films, coatings, elastomers, and foams. Films and coatings can be especially advantageous in protecting substrates from environmental weathering, chemical exposure, heat, and/or abrasion. Polyurethanes can also be engineered to be highly transparent and, if desired, can be formed into films and coatings with graphic arts for decorative applications.
Chemically, polyurethanes are distinguished by their characteristic carbamate (—NH—CO—O—) linkages and are generally prepared by reacting a multifunctional isocyanate with a multifunctional diol, or more generally polyol, with the presence of a catalyst. There are two general types of polyurethanes, thermoset and thermoplastic. Thermoset polyurethanes are highly crosslinked by covalent bonds. Thermoplastic polyurethanes are characterized by linear polymeric chains having self-ordering block structures. These polyurethanes are generally uncrosslinked but can also be lightly crosslinked. The block structures of a thermoplastic polyurethane generally include alternating “hard” and “soft” segments covalently bonded to each other end-to-end. The hard segments aggregate to form crystalline regions that act like physical crosslinks at ambient temperatures, but convert to a molten state upon heating. As a result, thermoplastic polyurethanes are well suited for thermoforming onto three dimensional articles and can be easily reprocessed.
Certain advantageous applications of polyurethanes relate to their use in hard coat applications. These include, for example, paint protection films or paint replacement films that protect the exterior surface of an automotive vehicle from harsh environmental conditions. Such conditions include impingement from stones, sand, debris, and insects, as well as general outdoor weathering, which can substantially degrade an automotive exterior over time. Composite polyurethane films have been previously described in U.S. Pat. No. 5,405,675 (Sawka et al.); U.S. Pat. No. 5,468,532 (Ho et al.); U.S. Pat. No. 6,607,831 (Ho); U.S. Pat. No. 6,383,644 (Fuchs); and International Patent Publication No. WO 2008/042883 (Ho et al.).